Cancer is one of the leading causes of death in the developed world, with over one million people diagnosed with cancer and 500,000 deaths per year in the United States alone. Overall it is estimated that more than 1 in 3 people will develop some form of cancer during their lifetime. There are more than 200 different types of cancer, four of which—breast, lung, colorectal, and prostate-account for over half of all new cases (Jemal et al., 2003, Cancer J. Clin. 53:5-26).
Folate Receptor 1 (FOLR1), also known as Folate Receptor-alpha, or Folate Binding Protein, is an N-glycosylated protein expressed on plasma membrane of cells. FOLR1 has a high affinity for folic acid and for several reduced folic acid derivatives. FOLR1 mediates delivery of the physiological folate, 5-methyltetrahydrofolate, to the interior of cells.
FOLR1 is overexpressed in vast majority of ovarian cancers, as well as in many uterine, endometrial, pancreatic, renal, lung, and breast cancers, while the expression of FOLR1 on normal tissues is restricted to the apical membrane of epithelial cells in the kidney proximal tubules, alveolar pneumocytes of the lung, bladder, testes, choroid plexus, and thyroid (Weitman S D, et al., Cancer Res 52: 3396-3401 (1992); Antony A C, Annu Rev Nutr 16: 501-521 (1996); Kalli K R, et al. Gynecol Oncol 108: 619-626 (2008)). This expression pattern of FOLR1 makes it a desirable target for FOLR1-directed cancer therapy.
Because ovarian cancer is typically asymptomatic until advanced stage, it is often diagnosed at a late stage and has poor prognosis when treated with currently available procedures, typically chemotherapeutic drugs after surgical de-bulking (von Gruenigen V et al., Cancer 112: 2221-2227 (2008); Ayhan A et al., Am J Obstet Gynecol 196: 81 e81-86 (2007); Harry V N et al., Obstet Gynecol Surv 64: 548-560 (2009)). Thus there is a clear unmet medical need for more effective therapeutics for ovarian cancers.
Three anti-FOLR1 antibodies have been examined as potential anti-cancer drugs. Murine monoclonal antibodies Mov18 and Mov19 were isolated in the late 1980s (Miotti S et al., Int J Cancer 39: 297-303 (1987)), confirmed to target FOLR1 (Coney L R et al., Cancer Res 51: 6125-6132 (1991)), and tested in pre-clinical studies for their ability to eradicate antigen-expressing cancer cells as conjugates with a cytotoxic ribosome-inactivating protein (Conde F P et al., Eur J Biochem 178: 795-802 (1989)).
Mov19 was tested as a bi-specific antibody targeting cytotoxic T cells and natural killer cells (Mezzanzanica D et al., Int J Cancer 41: 609-615 (1988); Ferrini S et al., In J Cancer Suppl 4: 53-55 (1989); Ferrini S et al., In J Cancer 48: 227-233 (1991)), and as a fusion protein of the single-chain Fv (scFv) of Mov19 with interleukin-2 in vivo (Melani C et al., Cancer Res 58: 4146-4154 (1998)). Chimeric (murine variable/human constant) anti-FOLR1 antibodies Mov18 and Mov19 have been examined pre-clinically on their ability to mediate cytotoxic immune cell-dependent killing of FOLR1-expressing tumor cells in vitro (Coney L R et al., Cancer Res 54: 2448-2455 (1994)), and a chimeric Mov18-IgE was tested in IgE-dependent immunotherapeutic preclinical models (Karagiannis S N et al., J Immunol 179: 2832-2843 (2007); Gould H J et al., Eur J Immunol 29: 3527-3537 (1999)).
Mov18 was studied in the form of conjugates with various radionuclides in preclinical studies and then, in early 1990s, in clinical trials (Zacchetti A et al., Nucl Med Biol 36: 759-770 (2009)), which ended without any drug being approved for clinical use.
MORAb003, a humanized form of the murine monoclonal anti-FOLR1 antibody LK26 was evaluated pre-clinically as a non-modified antibody (Ebel W et al., Cancer Immun 7:6 (2007)) and as a conjugate with the 111In radionuclide (Smith-Jones P M et al., Nucl Med Biol 35: 343-351 (2008)), and is currently undergoing clinical trials as a non-modified antibody (D. K. Armstrong et al. J. Clin. Oncol. 26: 2008, May 20 suppl; abstract 5500).